1. Field of the Invention
This invention relates to a device for timing detection of supply voltage in an electronic timepiece of the self-power charging type having a means for generating electric energy which is stored as power source in capacitors.
2. Destcription of the Prior Art
One example of the connection diagram of a charging circuit element of an electronic timepiece with charging function employed heretofore is shown in FIG. 2.
Since the present invention relates to the rising time control of charging, first an operation in the initial state of charging will be described with reference to FIG. 2.
In the first state of charging, all of switches a, b and c are opened. The switches a, b and c are comprised of MOS transistors, respectively, for example. Accordingly, a capacitor 2 having a small capacity is charged with electric energy generated by a solar cell 1. When the terminal voltage of the capacitor 2 rises, an integrated circuit 3 begins to operate. This state is determined as a first state. When the potential of a terminal Vc.sub.2 of the capacitor 2 exceeds a certain value after the integrated circuit begins to operate, the switch a is closed, and now the charging of a capacitor 4 of large capacity is started. This state is determined as a second state.
In the meantime, the integrated circuit 3 drives a step motor (not shown in the figure) to conduct a time keeping operation.
In the first and second states, accordingly, the integrated circuit 3 and the step motor are driven by the electric charge accumulated in the capacitor 2.
The capacity of the capacitor 2 used herein is set to be very small, about 6.8 .mu.F, for instance, for the purpose of the quick start of operation.
The shift from the first state to the second state occurs when the absolute value of the terminal voltage .vertline.Vc.sub.2 .vertline. of the capacitor 2 turns to be 2.0.sup.V or above, for instance. If the solar cell 1 is shaded from the incident light for several seconds after the rise of .vertline.Vc.sub.2 .vertline. above 2.0.sup.V brings about the shift to the second state (2) the generated current is ceased.
Then the terminal voltage .vertline.Vc.sub.2 .vertline. of the capacitor 2 falls to about 0.9.sup.V after a few times of driving of the step motor.
If no measures were taken in this condition, Vc.sub.2 would fall below the lowest operating voltage of the step motor, thereby causing the stop of operation or a failure in a rhythmical movement of the hand.
Even if the solar cell 1 were exposed again to the incident light thereafter, the terminal voltage .vertline.Vc.sub.2 .vertline. of the capacitor 2 would rise very slowly since the large-capacity capacitor 4 is connected to the load of the solar cell 1. Consequently the stopped state of the timepiece would continue a long time.
The capacity of the large-capacity capacitor 4 used herein is set to be about 0.3 F, for instance.
Accordingly, it would take about 10 minutes for the generated current of 200 .mu.A to raise the voltage of the large-capacity capacitor 4 from 0.9.sup.V to 1.3.sup.V, for instance, which enables the operation. During this period, the timepiece could not be restarted.
In order to prevent the occurrence of these problems, the second potential of Vc.sub.2 is detected in the state, and when Vc.sub.2 falls below a certain value, the switch a is opened to restore the first state.
As the result, the solar cell 1 turns to be loaded only with the small-capacity capacitor 2, and therefore the terminal voltage .vertline.Vc.sub.2 .vertline. of the capacitor 2 can be quickly raised in a short time.
Accordingly, such a switching operation is repeated between the first and second states in accordance with a balance between a generated energy and the consumed energy in the initial state of charging.
In this relation, the timing of detection of Vc.sub.2 in second the state is particularly important. FIG. 3 shows a prior-art detection timing chart.
FIG. 3 shows the timing of a driving pulse in a compensative driving system in which a compensating or correction driving pulse P.sub.2 of a large pulse width or large electric power is outputted when the step motor is not rotated by a main or normal driving pulse P.sub.1 of a small pulse width or small electric power.
Such a compensative driving system is utilized for the electronic timepiece of the self charging type in order to reduce power consumption.
In the prior-art, the timing of detection of voltage is controlled after the end of driving of the step motor, as shown by a voltage detection pulse 12 in FIG. 3 (the polarity of the waveform 12 means nothing in particular). Diodes 7 and 8 are reverse-current check diodes which check an ineffective current bypassing the integrated circuit 3.
The switch b and the switch c are used in an advanced state of charging. These switches b and c will not be described herein, because they have no direct relation with the description of the present invention.
In the case when the detection of voltage is conducted after a compensating driving pulse P.sub.2 is applied as shown in FIG. 3, it sometimes happens that the drive by the compensating pulse P.sub.2 11 can not be effected.
It is assumed, for instance, that the condition for the shift from the second state to the first state is .vertline.Vc.sub.2 .vertline..ltoreq.1.3.sup.V.
Furthermore, when .vertline.Vc.sub.2 .vertline. is 1.3.sup.V in the second and the pulse width of the main driving pulse P.sub.1 is set to 4 ms, .vertline.Vc.sub.2 .vertline. is lowered to about 1.05.sup.V after the output of the main driving pulse P.sub.1. If the step motor is not rotated by the normal pulse P.sub.1 on the occasion and the lowest driving voltage thereof is 1.2.sup.V, the step motor is also not rotated by the following compensating driving pulse P.sub.2 ; since the electric power of the compensating driving pulse P.sub.2 is decreased in proportion to the terminal voltage .vertline.Vc.sub.2 .vertline..
Thereafter, the condition .vertline.Vc.sub.2 .vertline..ltoreq.1.3.sup.V is detected by the voltage detection pulse 12 and the shift is made from the second state to the first state. Then the potential of Vc.sub.2 rises rapidly, so that an energy necessary for the following drives can be supplied. However, the first following drive also fails due to the failure of the preceding drive, thus resulting in a delay of two seconds.